Everyone should have at least two of these ESI 250DE Impedance bridges.
Well that was not my first thought when purchasing this unit on Ebay.
I was still having some issues with the “Internal DC” detector circuit on my existing unit, and the price of this unit was extremely reasonable. After dozens of hours troubleshooting the first unit with no solution, my thought was to compare the two circuits and salvage parts from the new unit if necessary.
Unit #2 arrived in what seemed to be good condition, with many years of dirt and dust buildup especially under the dials and knobs. As usual the first part of the project was to clean up the decades of grime and sticker residue.
Part two of the cleanup was a thorough cleaning of all switches and potentiometers with DeoxIt D5 contact cleaner, and lubrication of moving parts.
After the cleanup it was time to connect it up to my current limited bench power supply and do some initial testing.
Well, that was a disappointment. Nothing worked except for the battery test position.
After inspecting the main circuit board, I found the F3 module (black 13290) was very loose and wobbly. All the solder connections were in great shape with no signs of previous work, but I did notice that the top of the ground plane strip had an odd bend in it.
Upon de-soldering the F3 module I found that one of three leads had been sheared off leaving it disconnected from the board. After a quick repair I tried to reassemble the unit back in its case for testing with it’s 4 D‑cell battery pack, but couldn’t get the threads to line up with the mounting hole. Upon inspecting the internal mounting bracket, it was obvious that it had been severely bent and twisted.
It began to look like the unit had sustained a significant impact to cause this much bending, which also explained the sheared lead on the F3 module and damage to the ground plane strip.
The odd thing was that there was no external signs that this piece of test equipment had been dropped from such a height to cause this much damage.
After straightening the bracket and replacing a sheared screw the unit was tested again with the same results.
It was time to check everything for mechanical damage, but much to my surprise everything else other than the main circuit board was OK and all resistances were well within specifications.
It was time to de-solder the remaining two large items on the circuit board.
The F2 “Detector Network” module was in good shape and its resistance and capacitance measurements were within specifications.
The F1 “Input Filter” module did have a sheared pin, but I could not test the module due to a lack of documentation from the ESI manual.
My curiosity got the better of me so I decided to open the module to see how it worked.
After drilling out two dimples in the case which were mechanically retaining the circuit board, I was very pleased to find that the internals were not potted.
The F1 module circuit consists of 4 capacitors, 4 resistors, 2 diodes, and a shielded relay.
After testing the relay and all the other components, I documented the internals of all the F modules for future reference.
Modules F2 and F3 are a Twin‑T filter network and are used in the Detector and Generator circuits respectively.
Module F1 is a interesting circuit with pin 1 being the detector input which is connected through R1 a 100K Ohm resistor in the first stage of a RC filter to a diode limiter, C1 a 0.47 uF capacitor and R2.
The next RC filter stage is formed by R2 and C2, then R3 and C3 the next stage, then R4 and C4 the final stage before connecting to the reed relay contact and pin 4.
Pin 3 of module F1 is connected to ground when the mode is set to either of the DC Detector modes, tying one side of the four capacitors and the diode pair to ground.
When the ESI 250 DE is operating in the “Internal DC” mode the relay cycles at a 22 to 25 Hz rate connecting pin 4 to pin 5 at that same rate.
Well, after jumping down that rabbit hole of filter design and reverse engineering circuits. I found that after repairing the broken lead of the F1 module, the problem was that the circuit board card edge connector was not making good contact with socket connector fingers.
This was causing a several hundred Ohms resistance on one of the grounds. After a good cleaning with DeoxIt D5 and some delicate re-bending of the contact fingers the unit was operating correctly.
This cleaning and bending on unit #1 also corrected its “Internal DC” mode problem.
This unit also came with a model 1325 rechargeable battery cover that plugged directly into the existing battery fixture contacts in the battery compartment. Unfortunately the Ni-cad battery was not recoverable, so I am using standard D‑cell batteries without the charging circuit.
I now have two functional ESI 250DE Impedance Bridges. My favorite is still the Grey themed unit with it’s non-standard round meter. The grey themed unit has all plastic parts of the knobs, dials, and jacks in a light grey color with a medium grey faceplate, while the black themed unit has all it’s plastic pieces in black. The grey themed unit has it’s outer case painted with a grey-blue color with the other unit in a darker blue-grey color, and both units having a black plastic cover. My guess would be that the color schemes have to do with the age of when the units were produced. The most recent dated components in unit #2 are from early 1988, and unit #1 was built in 1971.
More internal pictures of unit #1 in the March 2018 blog post.
While most people do not need two of these ESI 250DE Impedance Bridges, I do recommend having at least one of these pieces of test equipment on your electronics bench for testing components and have found the 250DE to be extremely accurate.